Significant progress made on major California fire

 Firefighters watch as the Rocky fire burns near Clearlake, California, on Aug 1, 2015.
Firefighters watch as the Rocky fire burns near Clearlake, California, on Aug 1, 2015.EPA

LOS ANGELES (AFP) - The biggest fire raging in California has slowed considerably and, as of Thursday morning has been 40 per cent contained, officials said.

"Just in - Continued progress last night on #RockyFire - Now 69,600 acres and 40% contained. Over 3,600 firefighters fighting the fire," Daniel Berlant, spokesman for Cal Fire, the agency leading the firefighting effort, tweeted at 6.30am local time (9.30pm Singapore time).

At that acreage, about 28,000ha, the Rocky Fire is easily the largest so far this year in California and accounts for more than two-thirds of the combined area burned by all fires in that state in 2015.

The fire, approximately 160km north of San Francisco, has been burning since 29 July, stoked by extremely dry air, high temperatures and vegetation sucked of moisture over the course of a four-year drought.

Over the weekend, it burned 8,000ha on Saturday night alone.

As of Thursday morning, the blaze had destroyed 43 residences and threatens another 6,529 structures, according to Cal Fire, with 13,000 people forced to evacuate the area.

But a respite from higher temperatures and lower humidity over the past several days has allowed firefighting crews to make some headway ahead of higher temperatures and lightning storms expected later this week.

"Relative humidity recovery is contributing to diminished fire line activity," Cal Fire said in their incident report Thursday morning.

"With the return of critical weather parameters, increasing fire activity may be observed during the peak burning periods when temperatures are the highest and relative humidity is at its lowest.

The cause of the Rocky Fire is still under investigation, and while the majority of fires in northern California have been started by lighting, authorities appear to be increasing arrests.

Cal Fire arrested a suspected arsonist Wednesday and another, who had allegedly started 14 fires over the past week and a half, on Monday.

The vast Rocky Fire and numerous smaller ones burning in the state - as well as the death of a firefighter on Tuesday - caused California Governor Jerry Brown to declare a state of emergency Friday.

California and Colorado national guard planes have been called in to help contain the blaze.